1. Technical Field
The present invention relates to a plasma cell.
2. Prior Art
Different plasma cells are available on the market at the present time. These are:
hot filament cells which are cells of simple design which can be solely used to produce monoatomic hydrogen; diatomic hydrogen dissociating itself through the energy supplied by a filament brought to a high temperature;
radio frequency and cycloelectronic resonance plasma cells, which are cells requiring complicated, very costly manufacturing, and producing ionized species which may damage surfaces;
continuous discharge plasma cells.
One cell of this latter type, available on the market and described for example in the document referenced (1) at the end of this disclosure, is illustrated in FIG. 1. It is made up of a discharge tube 10 which curves backwards and is mounted on an ultravacuum flange 11 of CF 35 type enabling its fixation in a vacuum chamber 19, in which pressure is in the region of 10xe2x88x925 to 10xe2x88x926 mbar. This Figure illustrates a quartz tube 12, platinum electrodes 13 and their electrical connections 14, a thermal glass tube 15 and a tube in stainless steel 16. The upper part of the cell is fitted with a protective cover 17 and has a cone-shaped opening 18 through which the plasma can exit. A first flange 20 of CF 16 type provides for the addition of oxygen and a second flange 21 of the same type provides for connection to a pumping system.
A plasma is set up between the two electrodes 13 by applying a direct voltage of a few thousand volts, the current thus generated possibly varying from 20 mA to 40 mA. Ionization of the gas produces monoatomic species which exit through opening 18 and which come to react with sample 22 situated opposite the latter.
The major drawback of said cells lies in the presence of glass-metal soldering and their fragility, which means that they cannot be dismounted. These cells cannot be cleaned (deposits form on the cell walls) and therefore have a fairly short lifetime. Also, since the electrodes are not interchangeable, these cells are therefore xe2x80x9cmonogasxe2x80x9d cells and must be replaced for each new application, the creation of an atomic species requiring the association of a gas and a specific electrode: Pt for O2xe2x80x94Stainless steel for H2xe2x80x94Mo for N2 . . .
The purpose of the invention is to remedy the disadvantages of these cells of the prior art by providing a low-cost plasma cell, that is easy to maintain, having easily interchangeable electrodes and which can therefore be used with different types of gas.
The present invention relates to a plasma cell, able to be fixed into a vacuum chamber by means of a first flange, comprising an outer envelope in electric insulating material in which a gas under pressure is placed and provided in its upper part with a discharge opening, and electrodes, at least one anode and one cathode, arranged in said envelope and mounted on a second flange, at whose terminals a voltage can be applied such as to produce ionization of the gas producing the monoatomic species to be discharged through the opening, characterized in that it comprises electrodes that can be dismounted and are assembled on a second removable flange, electrically insulated separation means arranged between the cathode and anode or anodes, and a gas inlet opening.
In one advantageous embodiment, the envelope is in the shape of a glass bell. The separation means are formed of a quartz tube arranged around the cathode. The electrodes, which comprise several anodes, are arranged such as to allow electric selection of an anode. The gas inlet opening is located in the lower part of the envelope situated between the two flanges.
Advantageously, said cell can be used to create monoatomic species of different gases. Its lifetime is unlimited since it can be cleaned and the electrodes can be changed. Also, its manufacturing is much simplified.
It can be used to produce atomic species of gas from molecular sources such as hydrogen, oxygen, nitrogen, in a vacuum chamber. The monoatomic species formed in this way can, being highly reactive, be used either to dope or to change surface condition (cleaning, deoxidation, oxidation).